What to return when overriding Object.GetHashCode() in classes with no immutable fields?

Question 1

Go back to basics. You read my article; read it again. The two ironclad rules that are relevant to your situation are:

if x equals y then the hash code of x must equal the hash code of y. Equivalently: if the hash code of x does not equal the hash code of y then x and y must be unequal.
the hash code of x must remain stable while x is in a hash table.

Those are requirements for correctness. If you can't guarantee those two simple things then your program will not be correct.

You propose two solutions.

Your first solution is that you always return a constant. That meets the requirement of both rules, but you are then reduced to linear searches in your hash table. You might as well use a list.

The other solution you propose is to somehow produce a hash code for each object and store it in the object. That is perfectly legal provided that equal items have equal hash codes. If you do that then you are restricted such that x equals y must be false if the hash codes differ. This seems to make value equality basically impossible. Since you wouldn't be overriding Equals in the first place if you wanted reference equality, this seems like a really bad idea, but it is legal provided that equals is consistent.

I propose a third solution, which is: never put your object in a hash table, because a hash table is the wrong data structure in the first place. The point of a hash table is to quickly answer the question "is this given value in this set of immutable values?" and you don't have a set of immutable values, so don't use a hash table. Use the right tool for the job. Use a list, and live with the pain of doing linear searches.

A fourth solution is: hash on the mutable fields used for equality, remove the object from all hash tables it is in just before every time you mutate it, and put it back in afterwards. This meets both requirements: the hash code agrees with equality, and hashes of objects in hash tables are stable, and you still get fast lookups.

Question 2

I would either create an additional readonly field or else throw NotSupportedException. In my view the other option is meaningless. Let's see why.

Distinct (fixed) hash codes

Providing distinct hash codes is easy, e.g.:

class Sample
{
    private static int counter;
    private readonly int hashCode;

    public Sample() { this.hashCode = counter++; }

    public override int GetHashCode()
    {
        return this.hashCode;
    }

    public override bool Equals(object other)
    {
        return object.ReferenceEquals(this, other);
    }
}

Technically you have to look out for creating too many objects and overflowing the counter here, but in practice I think that's not going to be an issue for anyone.

The problem with this approach is that instances will never compare equal. However, that's perfectly fine if you only want to use instances of Sample as indexes into a collection of some other type.

Constant hash codes

If there is any scenario in which distinct instances should compare equal then at first glance you have no other choice than returning a constant. But where does that leave you?

Locating an instance inside a container will always degenerate to the equivalent of a linear search. So in effect by returning a constant you allow the user to make a keyed container for your class, but that container will exhibit the performance characteristics of a LinkedList<T>. This might be obvious to someone familiar with your class, but personally I see it as letting people shoot themselves in the foot. If you know from beforehand that a Dictionary won't behave as one might expect, then why let the user create one? In my view, better to throw NotSupportedException.

But throwing is what you must not do!

Some people will disagree with the above, and when those people are smarter than oneself then one should pay attention. First of all, this code analysis warning states that GetHashCode should not throw. That's something to think about, but let's not be dogmatic. Sometimes you have to break the rules for a reason.

However, that is not all. In his blog post on the subject, Eric Lippert says that if you throw from inside GetHashCode then

your object cannot be a result in many LINQ-to-objects queries that use hash tables internally for performance reasons.

Losing LINQ is certainly a bummer, but fortunately the road does not end here. Many (all?) LINQ methods that use hash tables have overloads that accept an IEqualityComparer<T> to be used when hashing. So you can in fact use LINQ, but it's going to be less convenient.

In the end you will have to weigh the options yourself. My opinion is that it's better to operate with a whitelist strategy (provide an IEqualityComparer<T> whenever needed) as long as it is technically feasible because that makes the code explicit: if someone tries to use the class naively they get an exception that helpfully tells them what's going on and the equality comparer is visible in the code wherever it is used, making the extraordinary behavior of the class immediately clear.

Question 3

Where I want to override Equals, but there is no sensible immutable "key" for an object (and for whatever reason it doesn't make sense to make the whole object immutable), in my opinion there is only one "correct" choice:

Implement GetHashCode to hash the same fields as Equals uses. (This might be all the fields.)
Document that these fields must not be altered while in a dictionary.
Trust that users either don't put these objects in dictionaries, or obey the second rule.

(Returning a constant value compromises dictionary performance. Throwing an exception disallows too many useful cases where objects are cached but not modified. Any other implementation for GetHashCode would be wrong.)

Where this runs the user into trouble anyway, it's probably their fault. (Specifically: using a dictionary where they shouldn't, or using a model type in a context where they should be using a view-model type that uses reference equality instead.)

Or perhaps I shouldn't be overriding Equals in the first place.

Question 4

If the classes truly contain nothing constant on which a hash value can be calculated then I would use something simpler than a GUID. Just use a random number persisted in the class (or in a wrapper class).

Question 5

A simple approach is to store the hashCode in a private member and generate it on the first use. If your entity doesn't change often, and you're not going to be using two different objects that are Equal (where your Equals method returns true) as keys in your dictionary, then this should be fine:

private int? _hashCode;

public override int GetHashCode() {
   if (!_hashCode.HasValue)
      _hashCode = Property1.GetHashCode() ^ Property2.GetHashCode() etc... based on whatever you use in your equals method
   return _hashCode.Value;
}

However, if you have, say, object a and object b, where a.Equals(b) == true, and you store an entry in your dictionary using a as the key (dictionary[a] = value).
If a does not change, then dictionary[b] will return value, however, if you change a after storing the entry in the dictionary, then dictionary[b] will most likely fail. The only workaround to this is to rehash the dictionary when any of the keys change.

What to return when overriding Object.GetHashCode() in classes with no immutable fields?

Background:

Question:

Summary:

Distinct (fixed) hash codes

Constant hash codes

But throwing is what you must not do!